Plasma process uniformity across a wafer by apportioning power among plural vhf sources

ABSTRACT

A method is provided for processing a workpiece in a plasma reactor chamber having electrodes including at least a ceiling electrode and a workpiece support electrode. The method includes coupling respective RF power sources of respective VHF frequencies f 1  and f 2  to either (a) respective ones of the electrodes or (b) a common one of the electrodes, where f 1  is sufficiently high to produce a center-high non-uniform plasma ion distribution and f 2  is sufficiently low to produce a center-low non-uniform plasma ion distribution. The method further includes adjusting a ratio of an RF parameter at the f 1  frequency to the RF parameter at the f 2  frequency so as to control plasma ion density distribution, the RF parameter being any one of RF power, RF voltage or RF current.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 60/898,632, filed Jan. 30, 2007.

BACKGROUND

Embodiments of the present invention concern a capacitively coupledplasma source for processing a workpiece such as a semiconductor wafer.A capacitively coupled plasma source comprises a ceiling electrode thatis driven at a very high frequency (VHF) frequency over 110 MHz whichcan produce a high density plasma at a relatively low voltage. Acapacitively coupled plasma source can further produce a low electrodepotential for low electrode erosion, and permits the ion energy at thewafer surface to be limited to a low level if desired, while operatingover a wide range of plasma density (very low to very high plasma iondensity). One problem inherent in such a plasma source is that theceiling electrode exhibits radial transmission line effects and loadingdue to the effective dielectric constant of the plasma. For example, at150 MHz, a free-space quarter wavelength is about 20 inches, which is onthe order of the diameter of the ceiling electrode (about 15 inches).Therefore, the RF field varies significantly across the surface of theceiling electrode, giving rise to process non-uniformities at the wafersurface. For a plasma with an effective dielectric constant greater than1, the effective wavelength is reduced to less than the ceilingelectrode diameter, worsening the non-uniformity of the RF field, makingprocessing non-uniformities across the wafer surface worse. For an etchprocess, this may produce a non-uniform edge low etch rate distributionacross the wafer surface.

Various approaches are employed to reduce such undesirable effects. Inone approach, magnetic steering may be employed to alter the plasma iondistribution, e.g., to reduce its center-high non-uniformity to producea somewhat flatter distribution. One problem with this approach is thata center-high non-uniformity of the source may be beyond the correctivecapability of magnetic steering. Another problem with this approach canbe electrical charging damage of the workpiece if the magnetic fluxdensity is too high. In another approach, the plasma sheath (or bias)voltage is increased by applying more plasma RF bias power to the wafer.This has the effect of increasing the plasma sheath thickness which inturn typically decreases the capacitance across the ceiling-plasmasheath as well as the capacitance across the wafer-plasma sheath,thereby forming three capacitors in series, including the ceiling sheathcapacitance, the plasma capacitance and the wafer sheath capacitance.The net effect is to reduce the effect of the dielectric constant of theplasma, thereby reducing the non-uniformity of the RF field. The highbias voltage required in some oxide etch plasma process recipes iscompatible with this latter approach. However, a high plasma biasvoltage is not desirable in some other types of plasma processes. Theworst non-uniformities appear in processes employing the lowest plasmabias voltage.

Such approaches are complicated by the fact that other processconditions dictated by the process recipe have as great an effect uponplasma distribution as either magnetic steering or bias (sheath)voltage. For example, increasing chamber pressure produces a less centerhigh and a more center low plasma ion distribution, while decreasing thechamber pressure produces a more center high distribution. Other changesin plasma distribution are caused by source power (plasma density), gaschemistry, electronegativity of the gas mixture, pumping rate, gas flowrate and other parameters dictated by the process recipe.

SUMMARY OF THE INVENTION

A method is provided for processing a workpiece in a plasma reactorchamber having electrodes including at least a ceiling electrode and aworkpiece support electrode. The method includes coupling respective RFpower sources of respective VHF frequencies f1 and f2 to either (a)respective ones of the electrodes or (b) a common one of the electrodes,where f1 is sufficiently high to produce a center-high non-uniformplasma ion distribution and f2 is sufficiently low to produce acenter-low non-uniform plasma ion distribution. The method furtherincludes adjusting a ratio of an RF parameter at the f1 frequency to theRF parameter at the f2 frequency so as to control plasma ion densitydistribution, the RF parameter being any one of RF power, RF voltage orRF current.

In one embodiment, f1 is greater than about 110 MHz and f2 is less thanabout 90 MHz. In a related embodiment, the adjusting includes reducing acenter-high plasma ion density distribution by decreasing the ratio ofthe RF parameter at the f1 frequency relative to the RF parameter at thef2 frequency. In another related embodiment, the adjusting includesreducing an edge-high plasma ion density distribution nonuniformity bydecreasing the ratio of the RF parameter at the f2 frequency relative tothe RF parameter at the f1 frequency.

In one embodiment, the method further includes providing respectivecenter ground return paths for RF current passing directly between theceiling electrode and the workpiece support electrode for thefrequencies f1 and f2 elements, and providing an edge ground return pathfor each of the frequencies f1 and f2. In another embodiment, the methodfurther includes adjusting the impedance of the center ground returnpath corresponding to the frequency f1 so as to increase or decrease thetendency of the RF power at f1 to produce a center-high non-uniformityin plasma ion density distribution.

In a further embodiment, the method includes adjusting the impedance ofthe center ground return path corresponding to the frequency f2 so as toincrease or decrease the tendency of the RF power at f2 to produce acenter-low or edge-high non-uniformity in plasma ion densitydistribution.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited embodiments of theinvention are attained and can be understood in detail, a moreparticular description of the invention, briefly summarized above, maybe had by reference to the embodiments thereof which are illustrated inthe appended drawings. It is to be noted, however, that the appendeddrawings illustrate only typical embodiments of this invention and aretherefore not to be considered limiting of its scope, for the inventionmay admit to other equally effective embodiments.

FIG. 1A illustrates a plasma reactor having multiple VHF source powerfrequencies applied to a ceiling electrode.

FIG. 1B depicts elements of a variable reactance or bandpass filtercontrolling the impedance of an RF ground return path in the reactor ofFIG. 1A.

FIG. 2 illustrates a plasma reactor having different VHF frequenciesapplied to opposing electrodes.

FIGS. 3A and 3B illustrate a plasma reactor with different VHFfrequencies applied to respective concentric electrodes.

FIG. 4 illustrates a plasma reactor with different VHF frequenciesapplied to the cathode electrode.

FIG. 5 illustrates a plasma reactor with two VHF source powerfrequencies, in which the high VHF source power frequency is producedusing a low VHF frequency generator and a third harmonic resonator.

FIG. 6 illustrates a plasma reactor with a single VHF variable frequencygenerator in the low portion (e.g., 50-60 MHz) of the VHF band with athird harmonic resonator to produce a VHF frequency component in thehigh portion (e.g., over 100 MHz) of the VHF band at a power leveldetermined by varying the generator output frequency.

FIG. 7 illustrates a process that can be carried out using the reactorof FIG. 1.

FIG. 8 illustrates a process that can be carried out using the reactorof FIG. 2.

FIG. 9 illustrates a process that can be carried out using the reactorof FIG. 3A.

FIG. 10 illustrates a process that can be carried out in the reactor ofFIG. 2 by setting the two VHF frequencies f1 and f2 of FIG. 2 equal toone another.

FIG. 11 illustrates a process that may be carried out using the reactorof FIG. 5.

FIG. 12 illustrates a process that can be carried out in a modificationof the reactor of FIG. 5 in which the locations of the f2 bandpassfilter 254 and the f2 generator and match 242, 246 are exchanged.

FIG. 13 illustrates a process that may be carried out in the reactor ofFIG. 6, using only a single lower VHF frequency generator.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. The drawings in the figures are all schematic and not toscale.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1A is a simplified schematic diagram of a plasma reactor capable ofcontrolling radial distribution of plasma ion density by apportioningcapacitively coupled plasma source power among different source powerfrequencies. The reactor has a vacuum chamber 200 enclosed by acylindrical side wall 202 and a disk-shaped ceiling 204. The ceiling 204is both a conductive ceiling electrode as well as a gas distributionshowerhead or plate, and will be referred to herein as the ceilingelectrode 204. The ceiling electrode may optionally be covered with aconducting, semiconducting or insulating material. The ceiling electrode204 includes inner and outer zones 206, 208 of gas injection orifices onits bottom surface 204 c coupled to respective inner and outer internalgas manifolds 210, 212. Inner and outer zone process gas supplies 214,216 furnish process gases to the inner and outer manifolds 210, 212. Awafer support pedestal 218 can support a workpiece such as asemiconductor wafer 220. The pedestal 218 may have the features of anelectrostatic chuck, including a conductive base layer 222 and aninsulating top layer 224 that encloses an internal electrode 226. Avacuum pump 228 is coupled through the floor 230 of the chamber 200. Thepedestal 218 is supported on a leg 232 that is coupled to a liftmechanism 234 that can elevate or depress the level of the pedestal 218.In one implementation, the lift mechanism 234 provides awafer-to-ceiling gap range from about 0.3inch to about 6 inches. Thewafer is clamped onto the pedestal by applying a D.C. clamping voltagefrom a D.C. supply 236 to the electrode 226. D.C. supply 236 typicallyincludes a low-pass filter to isolate the DC supply from the RF voltagepresent on the electrode 226. RF bias power may be coupled directly tothe internal electrode 226, or indirectly through the conductive baselayer 222. Pedestal 218 typically includes a conductive ground housing217 that is typically isolated from conductive base layer 222 andinternal electrode 226 by an insulating material such as quartz, ceramicor plastic. Alternatively, conductive base layer 218 may be grounded.

The uniformity of the plasma ion radial distribution across the chamber200 is controlled by providing a pair of VHF plasma source powergenerators 240, 242. In one aspect, the RF generator 240 has a frequencyin the upper portion of the VHF range, on the order of between 110 and250 MHz, and nominally about 162 MHz, while the other RF generator has afrequency in the lower portion of the VHF range, on the order of about40-90 MHz, and nominally about 60 MHz. We have discovered that thehigher VHF frequency from the generator 240 (if applied alone) tends toproduce a plasma ion density radial distribution that is center high andedge low, while the lower VHF frequency from the generator 242 (ifapplied alone) tends to produce a plasma ion density radial distributionthat is center low and edge high. In this respect, the two generatorscomplement one another when used simultaneously. In one embodiment, theoutput power of one of the generators 240, 242 are adjusted with respectto the one another to change the plasma ion density radial distributionbetween a center low pattern and a center high pattern. A selection ofthe ratio of the RF power (or voltage or current) levels of the twogenerators 240, 242 is made to minimize the center high and center lownon-uniformities and establish a more nearly uniform plasma iondistribution that is approximately free of both types ofnon-uniformities, and therefore nearly or substantially uniform. Suchuniformity may be determined by measuring the radial distribution ofetch rate across a wafer or workpiece. The variance of this distributiondecreases as uniformity increases. The variance for a more uniformradial distribution of etch rate may be as low as 4% or less, forexample.

In one embodiment, the higher VHF frequency generator 240 is coupled tothe ceiling electrode 204 through an impedance match network 244 thatmay be either fixed or dynamic and may be either formed of lumped ordistributed elements. The lower VHF frequency generator 242 is coupledto the ceiling electrode 204 through an impedance match network 246 thatis formed of either lumped or distributed elements and may be eitherfixed or dynamic. The output of the high VHF match 244 is protected fromthe output of the low VHF generator 242 by a notch filter 248 tuned toblock a narrow band centered around the frequency f2 of the low VHFgenerator 242, or alternatively by a high-pass filter tuned to block thefrequency f2 of the low VHF generator 242. The output of the low VHFmatch 246 is protected from the output of the high VHF generator 240 bya notch filter 250 tuned to block a narrow band centered around thefrequency f1 of the high VHF generator 240, or alternatively by alow-pass filter tuned to block the frequency f1 of the high VHFgenerator 240. The filter circuits are designed in accordance withconventional practice in conjunction with the matching networks so as toachieve the desired matching range with the required frequencyisolation.

Two RF ground return paths are provided for each of the VHF frequenciesf1, f2. A path along the side of the chamber 200 is provided bygrounding the side wall 202, as indicated in the drawing. VHF currentalong this path promotes an edge-high center low plasma ion radialdistribution, or at least a less center-high plasma ion radialdistribution relative to an RF ground return path through the center ofthe chamber. A path through the center of the chamber 200 is optionallyprovided by coupling the pedestal electrode 226 (or the base layer 222)to ground through respective tunable (variable) bandpass filters 252,254 which are controlled independently of one another. The variablebandpass filter 252 has a narrow pass band that includes (or is centeredat least approximately on) the frequency f1 of the higher VHF generator240. The variable bandpass filter 254 has a narrow pass band thatincludes (or is centered at least approximately on) the frequency f2 ofthe lower VHF generator 242. Both bandpass filters 252, 254 providerespective impedances to ground at their respective bandpass frequenciesf1, f2. These impedances are varied by a controller 270 to determine thedivision of RF current from each generator 240, 242 between the pedestalelectrode 226 and the side wall 202. The apportionment of this currentis controlled by varying the reactance of each bandpass filter 252, 254.Conventional RF filter circuits of capacitive and inductive componentsmay be employed to implement the variable bandpass filters 252, 254. Inaccordance with conventional practice, these filters may be implementedas lumped elements of capacitive and inductive components or asdistributed elements, such as coaxial tuning elements or stubs. Forexample, FIG. 1B is a simplified schematic diagram of a variablebandpass filter of the type that can be employed in the reactor of FIG.1A. The variable bandpass filter of FIG. 1B can include a shuntcapacitor 256, an inductor 258 and a load capacitor 260, either or bothcapacitors 256, 260 being variable. In accordance with one aspect, thefilters 252, 254 may not necessarily be bandpass filters or have thefrequency response of a bandpass filter. For example, one or both of thefilters 252, 254 may be a high pass filter or a low pass filter, or areactive element whose response can be varied to function as any type offilter. Alternatively, an RF ground return path through the center ofthe chamber 200 may be provided by grounding the pedestal electrode 226.This may be through a high-pass filter to permit effective isolation ofthe RF bias.

RF bias power is applied to the ESC electrode 226, including LF power(e.g., about 2 MHz) from a low frequency RF power generator 262 throughan LF impedance match 264, and HF power (e.g., about 13.56 MHz) from ahigh frequency RF power generator 266 through an HF impedance match 268.Typically, the RF bias frequencies are selected such that the LF powerlevel controls the peak ion energy, while the HF power level controlsthe central width of the ion energy distribution. An RF current groundpath may be provided for each of the RF bias sources applied to the ESCelectrode 226. A path through the ceiling 204 is optionally provided bycoupling the ceiling through a bandpass or low-pass filter to ground.Furthermore, a variable reactance may be inserted in the path to allowcontrol of the bias return current to the ceiling relative to biasreturn current to other surfaces, namely current to the wall 202 andring 219. The insertion reactance or impedance may be increased to forcemore bias return current to the edge (ring 219 or wall 202), which tendsto favor an edge high plasma ion density uniformity condition.Alternatively, the insertion reactance or impedance may be decreased toforce less bias return current to the edge (ring 219 or wall 202), whichtends to favor a center high plasma ion density uniformity condition.

The two VHF source power generators 240, 242 may be operated incontinuous wave (CW) mode or they may be pulsed synchronously orasynchronously with respect to one another. Moreover, either or both ofthe bias power generators 262, 266 may be operated in CW mode or in apulsed mode. In the pulsed mode, their duty cycles may be controlled tocontrol the time-averaged RF bias power or voltage (and therefore theion energy) at the wafer surface. The pulsing of the bias generators262, 266 may be synchronous or asynchronous with respect to each otherand/or with respect to the source power generators 240, 242. In thepulsed mode, any pair of the foregoing generators that are pulsedsynchronously to one another may have their RF envelopes coincident intime or offset in time and may be overlapping or non-overlapping.

Uniformity of gas flow across the surface of the wafer 220 anduniformity of the RF field near the wafer edge can be improved byproviding a below-wafer ground return 219 extending radially outwardlyfrom the side of the pedestal 218 at a level that is below the wafersupport surface of the pedestal 218. The below-wafer ground return 219is typically shaped as a cylinder or a flat annular ring that extendstoward the side wall 202 to form a gap 203 that partly constricts gasflow from the process region above the wafer into the pumping annulusbelow the wafer evacuated by the vacuum pump 228. The level of thebelow-wafer ground return is above features such as the wafer slit valve229 or pumping port that produces undesirable asymmetries in the plasmadistribution arising from asymmetries in gas flow pattern orelectrostatic or electromagnetic fields. The narrow gap between the sidewall and the outer edge of the below-wafer ground return 219 partiallyconstricts gas flow, such that the region above the wafer 220 is fairlyimmune to such asymmetries, thereby improving process uniformity. In oneimplementation, the below-wafer ground plane 219 is formed of aconductive material and is connected to ground. It therefore provides amore uniform ground reference at the wafer edge that renders theelectric field more uniform there and less susceptible to asymmetries inthe distribution of conductive surfaces in the chamber interior. Thering 219 may also serve as a plasma boundary to help confine the plasmavolume to the chamber region above the ring 219. In an alternativeimplementation, the ring 219 does not serve as a ground plane, and isinstead formed of a non-conductive material. In another alternativeimplementation, the ground return ring (or cylinder) 219 is at theworkpiece or wafer level or above workpiece level. It may be at or nearthe ceiling level and concentrically surround the ceiling electrode 204.In another embodiment, the level of the ground return ring 219 may beselectively adjusted relative to the workpiece level with a liftmechanism. For example, by attaching the ring 219 to the outside of thepedestal 218, the ring 219 is lifted up and down by the pedestal liftmechanism. The ground return ring 219 may be insulated from othergrounded surfaces in the chamber (such as the ESC base layer 224) so asto not be directly coupled to ground, and instead be coupled to groundthrough a variable reactive element (e.g., the variable filter 252). Inthis case, the ground return ring 219 serves as the edge ground returnpath for the VHF frequency f2. The height of this edge ground returnpath is therefore variable and serves as one of the adjustableparameters of the reactor.

A uniformity controller 270 controls the relative power output levels ofthe two VHF generators 240, 242 and optionally of the impedances of thevariable bandpass filters 252, 254. The controller 270 can set theimpedance of the high VHF frequency (f1) bandpass filter so as toprovide a lower impedance return path to ground through the wafer 220than the through the side wall 202 at the higher VHF frequency f1, sothat the power from the f1 generator 240 produces a more pronouncedcenter high radial distribution. Furthermore, the controller 270 can setthe impedance of the low VHF frequency (f2) bandpass filter so as toprovide a higher impedance return path to ground through the wafer 220than through the side wall 202 at the lower VHF frequency f2, so thatpower from the f2 generator 242 produces a more pronounced center lowand edge high radial distribution. The controller 270 apportions therelative power output levels of the high and low VHF frequencygenerators 240, 242 to either suppress a center high non-uniformity inetch rate distribution (by increasing the power output of the lower VHFfrequency generator 242) or suppress an edge high non-uniformity in etchrate distribution (by increasing the power outer of the higher VHFfrequency generator 240). The controller 270 may make such adjustmentsin response to non-uniformity patterns measured on apreviously-processed wafer by a downstream or in-line metrology tool272. During the processing of successive wafers, standard feedbackcontrol corrective techniques, implemented as programmed algorithms inthe controller 270, may be employed to enact successive corrections bythe uniformity controller 270 to minimize non-uniformities in etch ratedistribution sensed by the metrology tool 272. The metrology tool 272may be programmed to inform the controller 270 whether plasma iondensity distribution has a predominantly center-high non-uniformity or apredominantly edge-high non-uniformity. Alternatively, the metrologytool 272 may embody in-situ sensors may provide real-time signals to thecontroller 270. OES (optical emission spectroscopy) sensors may beplaced on the ceiling 204 at various radii, providing an indication ofradial plasma excited species density. The plasma itself may be used asthe light source, or external light sources may be used. Alternatively,interferometry sensors may be placed on the ceiling 204 at variousradii, providing an indication of workpiece film thickness rate ofchange as a function of radius. Alternatively, ion flux sensors may beplaced on the ceiling 204 at various radii, providing an indication ofradial plasma ion density. Alternatively, voltage sensors may be placedon the ceiling 204 at various radii, providing an indication of radialelectrode voltage. Alternatively, isolated voltage sensors may be placedon the ceiling 204 at various radii, providing an indication of radialplasma floating potential. Real-time control of plasma uniformity may beperformed by controller 270 using sensor input and conventionaltechniques.

The uniformity controller can also control the lift mechanism 234, inorder to provide another control dimension for improving uniformity ofplasma ion distribution (or uniformity of etch rate distribution). Byraising the pedestal 218 toward the ceiling electrode 204, thewafer-to-ceiling gap is decreased, which suppresses plasma ion densitynear the center of the wafer and promotes plasma ion density near thewafer edge. Conversely, by lowering the pedestal 218 away from theceiling electrode 204, the wafer-to-ceiling gap is increased, whichpromotes plasma ion density over the wafer center while detracting fromplasma ion density at the wafer edge. Thus, the plasma distribution maybe rendered more center-high or more center-low by raising or loweringthe pedestal 218, respectively. As discussed above, the plasmadistribution may be rendered more center-high or more center-low byincreasing or decreasing, respectively, the ratio of the higher VHFfrequency power to lower VHF frequency power. Thus, the pedestal heightand the VHF power ratio are two different controls that affect theplasma ion distribution. The uniformity controller 270 can employ bothof these controls simultaneously to optimize plasma ion distributionuniformity. For example, an edge-high plasma non-uniformity may bereduced by increasing the output power of the higher VHF frequencygenerator 240, which may tend to increase a center-high peak in plasmaion distribution. This increase in the center-high peak may besuppressed, without requiring further change in the VHF powerapportionment, by raising the pedestal 218 to decrease the wafer-ceilinggap until an optimum plasma distribution is realized. This may be usefulfor process recipes calling for a low RF bias and a low chamberpressure, in which case the center-high peak in plasma ion distributionis particularly pronounced. The control of both VHF frequencyapportionment together with control of the wafer-ceiling gap extends therange of non-uniformity that the controller 270 is capable ofcounteracting. For a severe center-high nonuniformity, for example, thecontroller 270 may call for both an increase in the higher-versus-lowerVHF frequency power apportionment as well as a narrower wafer-ceilinggap.

The variable wafer-to-ceiling gap affects where a particular VHFfrequency (e.g., f1 or f2) has a peak in non-uniform plasma ion densitydistribution. Therefore, the controller 270 can set the gap to optimizethe choice of f1 to produce a predominantly center-high non-uniformplasma ion density distribution and the choice of f2 to produce apredominantly edge-high non-uniform plasma ion density distribution. Forexample, the controller 270 sets the wafer-ceiling gap to optimize thechoice of f1 and f2 to produce the different non-uniformity patterns,and the controller 270 varies the ratio of RF power (or current orvoltage) at the different frequencies f1, f2 to control the plasma iondistribution and reduce its non-uniformities.

The controller 270 may respond to an indication from the metrology tool272 of a predominantly center-high or edge-high non-uniformity in plasmaion density distribution by measuring and controlling (changing) any oneof the following so as to tend to reduce that non-uniformity: (a) theratio of RF voltages at the frequencies 1f1, f2; (b) the ratio of RFcurrents at the frequencies f1, f2; or (c) the ratio of RF power at thefrequencies f1, f2. Such measurements may be made at the respectiveelectrodes, for example, or another suitable location.

In one alternate mode, the controller 270 varies plasma ion densitydistribution without necessarily changing the apportionment of poweramong the higher (f1) and lower (f2) VHF generators 240, 242. Instead,plasma ion density distribution is changed by the controller 270 byvarying the impedances to the center ground return paths presented bythe f1 and f2 variable bandpass filters 252, 254. For example, thetendency of the higher frequency (f1) VHF power to create a center peakor suppress an edge peak in plasma density distribution may be increasedor decreased by changing the impedance presented to the f1 power by thevariable bandpass filter 252. Likewise, the tendency of the lowerfrequency (f2) VHF power to create an edge peak or suppress a centerpeak in plasma ion density distribution may be increased or decreased bychanging the impedance presented to the f2 power by the variablebandpass filter 254. Such changes affect the apportionment of VHFcurrent at each of the frequencies f1, f2 between the center groundreturn path (ceiling-to-wafer) and the side ground return path (throughthe side wall 202). By directing more of the f1 power to the centerground return path, the tendency of the higher VHF frequency (f1) powerto create a center-high distribution is increased. By directing more ofthe f2 power to the side ground return path, the tendency of the lowerVHF frequency (f2) power to create an edge-high distribution isincreased. In some cases, the controller may change the ground returnpath apportionment for only one of the two frequencies f1, f2.

In a further alternate mode of the reactor of FIG. 1, only one of theVHF generators (e.g., only the generator 240) provides RF power, theother generator (e.g., the generator 242) not being used or else beingeliminated. The uniformity controller 270 changes the plasma ion radialdistribution by varying the f1 bandpass filter 252 so as to control theimpedance of the ground return path through the ESC electrode 226. Thisapportions the ground return currents between the center path throughthe ESC electrode 226 and the side path through the side wall 202. As aresult, this feature of the controller 270 varies the center-high andcenter-low non-uniformities in plasma ion distribution (or equivalentlyin etch rate distribution) to optimize uniformity.

While only two VHF generators 240, 242 are illustrated in FIG. 1A, moreVHF generators may be employed of different frequencies. For example, athird VHF generator may be employed having a frequency higher thaneither of the two VHF generators 240, 242. As described above, the highVHF frequency generator (e.g., 162 MHz) produces a center peak in plasmaion distribution while the lower frequency generator 242 (60 MHz)produces an edge peak. Uniformity may be improved by introducing a thirdVHF generator having an even higher frequency that produces peaksbetween the center and edge that fill in the minima in the plasma iondensity radial distribution.

The reactor of FIG. 1A may be used to reproduce plasma processconditions characteristic of a very low density bias-only plasmaconventionally produced with a single HF (13.56 MHz) frequency source toboth generate plasma ions and control the bias voltage on the wafer.This simulation may be realized by applying only an LF (e.g., 2 MHz)bias power from the generator 264, and setting the output power of eachof the two VHF generators 240, 242 to a very low level (e.g., 10 Watts)to establish the low plasma ion density desired. The advantage of thisis that the two generators 240, 242 may be adjusted with very finechanges in output power to maintain plasma uniformity over a far widerrange of changing process conditions than would be achievable with asingle HF (13.56 MHz) frequency source.

FIG. 2 depicts a modification of the reactor of FIG. 1A, in which thelower VHF frequency (f2) generator 242 and its match 246 and notchfilter 250 are coupled to the ESC electrode 226 rather than the ceilingelectrode 204. In this case, the f2 ground return path is through theceiling electrode 204. Therefore, the f2 variable bandpass filter 254 iscoupled to the ceiling electrode 204 rather than the ESC electrode 226.A notch filter 255 tuned to block RF current from the higher VHFfrequency (f1) generator 240 may be connected to the f2 bandpass filter254. Likewise, a notch filter 253 tuned to block RF current from thelower VHF frequency (f2) generator 242 may be connected to the f1bandpass filter 252.

In one alternative mode of the reactor of FIG. 2, the VHF frequencies f1and f2 applied to the top (ceiling electrode 204) and bottom (ESCelectrode 226) respectively are the same frequency (f1=f2). In thiscase, the controller 270 varies radial distribution of ion density (oretch rate) by varying the phase between the voltages (or currents) atthe ceiling electrode 204 and the ESC electrode 226. The phase betweenthe currents at the ceiling electrode 204 and the ESC electrode 226 maybe controlled, for example, by varying the reactance of the bandpassfilters 252, 254. Alternatively, the phase may be controlled at one orboth generators 240, 242. For example, if the reactances of the bandpassfilters 252, 254 are the same (and if there are no other differences),then the phase angle between the RF currents at the ceiling and ESCelectrodes 204, 226 is zero. At a phase of 180 degrees, essentially allof the current flows between the ceiling electrode 204 and the ESCelectrode 226, generating a center-high distribution of plasma iondensity or etch rate. At a phase of zero degrees, essentially all of thecurrent flows from either the ceiling electrode 204 or the ESC electrode226 to the side wall 202, generating a center-low edge-highdistribution. Therefore, the controller 270 can vary the phase anglebetween 0 and 180 degrees to attain a wide range of results.

In another alternate mode of the reactor of FIG. 2, only one of the VHFgenerators (i.e., only the f2 generator 242) provides RF power, theother generator 240 not being used or else being eliminated. Theuniformity controller 270 changes the plasma ion radial distribution byvarying the f2 bandpass filter 254 so as to control the impedance of theground return path through the ceiling electrode 204, so that itincreases or decreases relative to the (fixed) impedance of the groundreturn path through the side wall 202. This apportions the ground returncurrent between the center path through the ceiling electrode 204 andthe side path through the side wall 202. As a result, this feature ofthe controller 270 varies the center-high and center-lownon-uniformities in plasma ion distribution (or equivalently in etchrate distribution) to optimize uniformity.

In yet another alternate mode of the reactor of FIG. 2, only one of theVHF generators (i.e., only the f1 generator 240) provides RF power, theother generator 242 not being used or else being eliminated. Theuniformity controller 270 changes the plasma ion radial distribution byvarying the f2 bandpass filter 252 so as to control the impedance of theground return path through the ESC electrode 226, so that it increasesor decreases relative to the (fixed) impedance of the ground return paththrough the side wall 202. This apportions the ground return currentbetween the center path through the ESC electrode 226 and the side paththrough the side wall 202. As a result, this feature of the controller270 varies the center-high and center-low non-uniformities in plasma iondistribution (or equivalently in etch rate distribution) to optimizeuniformity.

FIGS. 3A and 3B depict a modification of the reactor of FIG. 1 in whichthe ceiling electrode 204 is divided into radially inner and outersections 204 a, 204 b that are electrically isolated from one another,and separately driven by respective ones of the generators 240, 242.While either generator may be selected to drive the inner electrode 204a leaving the other to drive the outer electrode 204 b, it is preferredthat the higher VHF frequency generator 240 be coupled to the innerelectrode 204 a and the lower VHF frequency generator 242 be coupled tothe outer electrode 204 b, in order to enhance the tendency of thehigher frequency to develop a center-high ion distribution and enhancethe tendency of the lower frequency to develop a center-low iondistribution.

FIG. 4 depicts a modification of the reactor of FIG. 1 in which both theVHF generators 240, 242 drive the ESC electrode 226 while the groundreturn bandpass filters 252, 254 are coupled to the ceiling electrode204.

FIG. 5 depicts a modification of the reactor of FIG. 2, in which the twofrequencies f1 and f2 are both in the lower portion of the VHF band. Forexample, f1 and f2 may be 54 MHz and 60 MHz, respectively. Thisrepresents a significant cost savings by eliminating the need for a highVHF frequency generator having an output frequency near 200 MHz or over150 MHz. In the reactor of FIG. 5, the missing high VHF frequency (e.g.,162 MHz), that provides the center-high response, is produced with ahigh VHF frequency (e.g., 162 MHz) resonator 274 coupled to the ceilingelectrode 204 (or alternatively to the output of the f1 generator 240).Preferably, the resonator 274 is tuned to resonate at an odd harmonic off1, such as the third harmonic. For example, if f1=54 MHz, then thethird harmonic generated in the resonator 274 would be 162 MHZ.Production of the higher harmonic is facilitated by the non-linearresponse of the plasma in the reactor chamber that functions as afrequency multiplier in cooperation with the resonator 274. The variablebandpass filter 252 is tuned to the third harmonic of f1 so that some ofthe RF power at f1 from the generator 240 is converted to the thirdharmonic of f1.

In another alternate mode of the reactor of FIG. 5, only one of the VHFgenerators (i.e., only the generator 240) provides RF power, the othergenerator 242 not being used or else being eliminated. The uniformitycontroller 270 changes the plasma ion radial distribution by varying thef1 bandpass filter 252 so as to control the impedance of the groundreturn path through the ceiling electrode 204, so that it increases ordecreases relative to the (fixed) impedance of the ground return paththrough the side wall 202. This apportions the ground return currentsbetween the center path through the ceiling electrode 204 and the sidepath through the side wall 202. As a result, this feature of thecontroller 270 varies the center-high and center-low non-uniformities inplasma ion distribution (or equivalently in etch rate distribution) tooptimize uniformity.

FIG. 6 depicts a modification of the reactor employing simultaneous highand low VHF frequencies but employing only a single low VHF frequencygenerator to achieve a great cost savings. The low VHF generator 240 isa variable frequency oscillator (VFO) whose frequency is varied by thecontroller 270 between a fundamental frequency f and f±Δf, where Δf is asmall deviation from f. The resonator 274 is tuned to the thirdharmonic, F=3·f, of the fundamental frequency f. By changing thefrequency of the generator 240, the proportion of the output power ofthe generator that is converted to the third harmonic F is increased ordecreased in inverse proportion to the difference between the generatoroutput frequency f±Δf and the fundamental frequency f whose thirdharmonic is the resonant frequency of the resonator 274. The result isthat both frequencies, i.e., the generator output frequency f±Δf and theharmonic frequency F, are coupled to the plasma, and their relativepower levels are controlled by varying the output frequency of thegenerator 240. By decreasing the difference between the generator outputfrequency and the fundamental frequency f, the power coupled to theplasma at the third harmonic increases while the power at thefundamental, f, decreases, thereby increasing the center-highnon-uniformity or decreasing the edge-high non-uniformity. Conversely,by increasing the difference between the generator output frequency andthe fundamental frequency f, the power coupled to the plasma at thethird harmonic decreases while the power at the fundamental, f,increases, thereby increasing the edge-high non-uniformity or decreasingthe center-high non-uniformity. Therefore, plasma uniformity isregulated by the controller 270 by varying the frequency of the VFO orgenerator 240. The two variable bandpass filters 252, 254 have passbandscentered at, respectively, the fundamental, f, and the third harmonic,F.

In one aspect, the interior chamber elements are formed of a metal suchas aluminum. In order to prevent or minimize metal contamination duringplasma processing, the surfaces of the metal chamber elements that canbe exposed to plasma, such as the interior surface of the side wall 202and the exposed surfaces of the pedestal 218, are coated with a film ofa process-compatible material, such as yttria, for example. The film maybe a plasma-spray-coated yttria. Alternatively, bulk ceramic materialsuch as yttria may be bonded to underlying metal interior chamberelements. For example, the ceiling 204 may have a bonded ceramic plateon the side exposed to plasma. The sidewall 202 may include a bondedceramic cylinder on the side exposed to plasma, or the ring 219 mayinclude a bonded ceramic ring on the side exposed to plasma. Ceramicmaterials may be doped or otherwise fabricated such that theirelectrical resistivity is in the semiconducting range (e.g., resistivityin the range 10̂8 to 10̂12 ohm*cm) to provide a DC current return path forthe ESC clamping voltage applied to the ESC electrode 226. These chambersurfaces may be heated in order to minimize undesired deposition oraccumulation of materials such as polymers, for example, or cooled tominimize or eliminate etching, or temperature controlled employing bothheating and cooling. The interior surfaces of the chamber may be cleanedin a plasma etch process by employing an appropriate chemistry. Forexample, in a dry cleaning step, oxygen or oxygen-containing, orchlorine or chlorine-containing gas may be introduced into the chamberand a plasma may generated using the VHF source power generators 240,242 and/or the bias power generators 262, 266.

FIG. 7 illustrates a process that can be carried out using the reactorof FIG. 1. In block 300 of FIG. 7, RF plasma source power iscapacitively coupled through an electrode (ceiling or wafer) at twodifferent VHF frequencies f1 and f2 simultaneously, where f1 is in thehigher range of the VHF band (e.g., 162 MHz) and f2 is in the lowerregion of the VHF band (e.g., 50-60 MHz). In block 302, an individualcenter ground return path is provided through a counter electrode (waferor ceiling) for each of the frequencies f1 and f2, by providing thebandpass filters 252, 254 to ground as shown in FIG. 1. In block 304 ofFIG. 7, an edge return path is provided through the side wall for eachof the frequencies f1 and f2 by grounding the side wall 202 as shown inFIG. 1. In block 306, the impedance of the f1 center return path isadjusted relative to the impedance of the f1 edge return path to promotecurrent flow at the f1 frequency to the center return path, by adjustingthe bandpass filter 252. In block 308, the impedance of the f2 edgereturn path is adjusted relative to the impedance of the f2 centerreturn path to promote current flow at the f2 frequency to the sidewall, by adjusting the bandpass filter 254. In block 310, the uniformitycontroller 270 improves the uniformity of the radial plasma ion densitydistribution by selecting a ratio of VHF power at the f1 frequency toVHF power at the f2 frequency. The step of block 310 may be carried outto reduce a center-high plasma ion density distribution by decreasingthe ratio of VHF power at the f1 frequency relative to VHF power at thef2 frequency (block 312). Or, the step of block 310 may be carried outto reduce an edge-high plasma ion density distribution nonuniformity bydecreasing the ratio of VHF power at the f2 frequency relative to VHFpower at the f1 frequency (block 314). As another way of affecting orimproving ion density distribution, the controller 270 may adjust theimpedances of the center and edge return paths of either or both f1 andf2 (by adjusting the respective bandpass filters 252, 254) to either:(a) channel more current toward the edge in order to suppress acenter-high non-uniformity or (b) channel more current toward the centerto suppress an edge-high non-uniformity (block 316).

In this description, uniformity may be referred to with respect toradial plasma ion density distribution. It is understood that such adistribution is inferred from or is equivalent to etch rate radialdistribution that can be measured across the surface of a wafer that hasbeen processed by a plasma etch process in the reactor.

FIG. 8 illustrates a process that can be carried out using the reactorof FIG. 2. In the step of block 318 of FIG. 8, RF plasma source power iscapacitively coupled through one electrode (ceiling or wafer) at anupper VHF frequency f1 (e.g., about 162 MHz) while RF plasma sourcepower is capacitively coupled through the counterelectrode (wafer orceiling) at a lower VHF frequency f2 (e.g., about 50-60 MHz). In block320, a center return path is provided through the counterelectrode forthe frequency f1. In block 322, a center return path is provided throughthe electrode for the frequency f2. In the step of block 324, an edgereturn path through the side wall for each of the frequencies f1 and f2.In the step of block 326, the impedance of the f1 center return path isadjusted relative to the impedance of the f1 edge return path to promotecurrent flow at the f1 frequency to the center return path, by adjustingthe variable bandpass filter 252. In the step of block 328, theimpedance of the f2 side return path is adjusted relative to theimpedance of the f2 center return path to promote current flow at the f2frequency to the side wall, by adjusting the variable bandpass filter254. In the step of block 330, the controller 270 improves theuniformity of the radial plasma ion density distribution by selecting aratio of VHF power at the f1 frequency to VHF power at the f2 frequency.This step may be carried out to reduce center-high plasma ion densitydistribution by decreasing the ratio of VHF power at the f1 frequencyrelative to VHF power at the f2 frequency (block 332). This step may becarried out to reduce edge-high plasma ion density distributionnonuniformity by decreasing the ratio of VHF power at the f2 frequencyrelative to VHF power at the f1 frequency (block 334). Alternatively orin addition to the step of block 330, the controller 270 may improveuniformity by adjusting the impedances of the center and edge returnpaths of either or both f1 and f2 (by adjusting the respective bandpassfilters 252, 254) to either: (a) channel more current toward the edge inorder to suppress a center-high non-uniformity or (b) channel morecurrent toward the center to suppress an edge-high non-uniformity (block336 of FIG. 8).

FIG. 9 illustrates a process that can be carried out using the reactorof FIG. 3A. In the process of FIG. 9, RF plasma source power through aninner ceiling electrode at an upper VHF frequency f1 RF plasma sourcepower is capacitively coupled through an outer ceiling electrode atlower VHF frequency f2 (block 338 of FIG. 9). In block 340, a centerreturn path is provided through the wafer for the frequency f1 byproviding the bandpass filter 252 coupled to ground. In block 342, acenter return path through the wafer is provided for the frequency f2 byproviding the bandpass filter 254 coupled to ground. In block 344 ofFIG. 9, an edge return path through the side wall 202 for each of thefrequencies f1 and f2 by grounding the side wall 202, as shown in FIG.3A. In the step of block 346, the impedance of the f1 center return pathis adjusted relative to the impedance of the f1 edge return path topromote current flow at the f1 frequency to the center return path, byadjusting the reactance of the bandpass filter 252. In the step of block348, the impedance of the f2 edge return path is adjusted relative tothe impedance of the f2 center return path to promote larger currentflow at the f2 frequency to the side wall, by adjusting the reactance ofthe bandpass filter 254. In block 350, the controller 270 improves theuniformity of the radial plasma ion density distribution (or of etchrate distribution on the wafer) by selecting a ratio of VHF power at thef1 frequency to VHF power at the f2 frequency. This step may be carriedout to reduce a center-high plasma ion density distribution bydecreasing the ratio of VHF power at the f1 frequency relative to VHFpower at the f2 frequency (block 352). Or, this step may be carried outto reduce edge-high plasma ion density distribution nonuniformity bydecreasing the ratio of VHF power at the f2 frequency relative to VHFpower at the f1 frequency (block 354). Alternatively, or in addition tothe step of block 350, the controller 270 may improve uniformity ofplasma ion density distribution (or etch rate distribution on the wafer)by adjusting the impedances of the center and edge return paths ofeither or both f1 and f2 to either: (a) channel more current toward theedge in order to suppress a center-high non-uniformity or (b) channelmore current toward the center to suppress an edge-high non-uniformity(block 356 of FIG. 9).

FIG. 10 illustrates a process that can be carried out in the reactor ofFIG. 2 by setting the two VHF frequencies f1 and f2 of FIG. 2 equal toone another (or at least nearly equal to one another). The bandpassfilters 252, 254 are used in this case as variable reactances that cancontrol or vary the phase between the VHF voltages (or currents) at theceiling and wafer. In the step of block 358 of FIG. 10, RF plasma sourcepower is capacitively coupled through one electrode (ceiling or wafer)at a VHF frequency while capacitively coupling RF plasma source powerthrough the counterelectrode (wafer or ceiling) at the same VHFfrequency. In block 360, a control element such as a variable reactance(e.g., the variable bandpass filter 252) is provided at thecounterelectrode 226 of FIG. 2 for controlling phase. In block 362, acontrol element such as a variable reactance (e.g., the variablebandpass filter 254) is provided at the electrode 204 for controllingphase. In the step of block 364, an edge return path is provided bygrounding the side wall 202. In the step of block 366, the controller270 improves the uniformity of the radial plasma ion densitydistribution by controlling the phase difference between the VHFcurrents at the electrode and the counterelectrode. This step may becarried out to reduce a center-high plasma ion density distribution bymoving the phase difference toward 180 degrees (block 367 of FIG. 10).Or, the step of block 368 may be carried out to reduce an edge-highplasma ion density distribution nonuniformity by moving the phasedifference towards 0 degrees.

FIG. 11 illustrates a process that may be carried out using the reactorof FIG. 5. In block 370 of FIG. 11, RF plasma source power at twosimilar VHF frequencies f1 and f2, both in the lower region of the VHFband, to an electrode (204 of FIG. 5) and to a counterelectrode (226 ofFIG. 5), respectively. This represents a significant cost savings byeliminating the cost of a high VHF frequency (e.g., 160-200 MHz)generator. In block 372 of FIG. 11, the electrode 204 is coupled to aresonator (274 of FIG. 5) having a resonant frequency which is an odd(e.g., third) harmonic of f1, and lies in the higher region of the VHFband, so as to produce VHF power at the odd (e.g., third) harmonic(e.g., 162 MHz). In block 374, an individual center return path isprovided through the counter electrode (266 of FIG. 5) for the thirdharmonic of f1, for example by providing the bandpass filter 252. Inblock 376, an individual center return path is provided through theelectrode 204 for the VHF frequency f2, for example by providing thebandpass filter 254. In block 378, an edge return path is providedthrough the side wall for f2 and for the odd harmonic of f1, bygrounding the side wall (202 of FIG. 5). In the step of block 380, thecontroller 270 adjusts the impedance of the f1 harmonic center returnpath relative to the impedance of the f1 harmonic edge return path topromote current flow at the f1 harmonic to the center return path, byadjusting the reactance of the bandpass filter 252. In the step of block382, the controller 270 adjusts the impedance of the f2 edge return pathrelative to the impedance of the f2 center return path to promotecurrent flow at the f2 frequency to the side wall, by adjusting thereactance of the bandpass filter 254. The controller 270 improves theuniformity of the radial plasma ion density distribution by selecting aratio of VHF power between the f1 and f2 generators to control the ratiobetween the f1 harmonic power and f2 power coupled to the plasma (block384). This step may be carried out to reduce a center-high plasma iondensity distribution by decreasing the ratio of VHF power generated atthe f1 frequency relative to VHF power at the f2 frequency (block 386).Or, this step may be carried out to reduce edge-high plasma ion densitydistribution nonuniformity by decreasing the ratio of VHF power at thef2 frequency relative to VHF power generated at the f1 frequency (block388 of FIG. 11). Alternatively or in addition to the step of block 384,the controller 270 may improve uniformity of plasma ion densitydistribution by adjusting the impedances of the center and edge returnpaths of either or both f2 and the harmonic of f1 to either: (a) channelmore current toward the edge in order to suppress a center-highnon-uniformity or (b) channel more current toward the center to suppressan edge-high non-uniformity (block 390).

FIG. 12 illustrates a process that can be carried out in a modificationof the reactor of FIG. 5 in which the locations of the f2 bandpassfilter 254 and the f2 generator and match 242, 246 are exchanged, sothat both frequencies f1, f2 drive the ceiling electrode 204. At block392, RF plasma source power at two similar lower VHF frequencies f1 andf2 simultaneously to an electrode (e.g., the ceiling electrode 204 ofFIG. 5). At block 394, a resonator (274 of FIG. 5) is coupled theelectrode 204, the resonator having a resonant frequency which is an oddharmonic of f1, so as to produce VHF power at the odd harmonic. Thisfrequency up-conversion is facilitated by the non-linear response of theplasma that provides a frequency-multiplying effect. In block 396 ofFIG. 12, an individual center return path is provided through a counterelectrode (226 of FIG. 5) for the harmonic of f1, by providing thebandpass filter 252 coupled to ground. In block 398 of FIG. 12, anindividual center ground return path is provided through thecounterelectrode for the VHF frequency f2 by providing the bandpassfilter 254 of FIG. 5 coupled to ground. In block 400, edge return pathsare provided through the side wall for f2 and the harmonic of f1, bygrounding the side wall 202 of FIG. 5. In block 402, the controller 270adjusts the impedance of the f1 harmonic center return path relative tothe impedance of the f1 harmonic edge return path to promote currentflow at the f1 harmonic through the center return path, by adjusting thebandpass filter 252 of FIG. 5. In block 404 of FIG. 12, the controller270 adjusts the impedance of the f2 edge return path relative to theimpedance of the f2 center return path to promote current flow at the f2frequency to the side wall, by adjusting the reactance of the bandpassfilter 254 of FIG. 5. In block 406, the controller 270 improve theuniformity of the radial plasma ion density distribution by selecting aratio of VHF power between the f1 and f2 generators to control the ratiobetween the f1 harmonic power and f2 power coupled to the plasma. Thisstep may be carried out to reduce center-high plasma ion densitydistribution by decreasing the ratio of VHF power at the f1 harmonicrelative to VHF power at the f2 frequency (block 408). Or, this step maybe carried out to reduce edge-high plasma ion density distributionnonuniformity by decreasing the ratio of VHF power at the f2 frequencyrelative to VHF power at the f1 harmonic (block 410). Alternatively orin addition to the process at block 408, the controller 270 may improveuniformity by adjusting the impedances of the center and edge returnpaths of either or both the f1 harmonic and f2, to either: (a) channelmore current toward the edge in order to suppress a center-highnon-uniformity or (b) channel more current toward the center to suppressan edge-high non-uniformity (block 412 of FIG. 12).

FIG. 13 illustrates a process that may be carried out in the reactor ofFIG. 6, using only a single lower VHF frequency generator (between about50-60 MHz) to realize the functionality that requires two generators inthe reactors described previously herein. In block 414 of FIG. 13, RFplasma source power is capacitively coupled through an electrode (e.g.,the ceiling electrode 204 of FIG. 6) from a variable frequency VHFgenerator 240 having a frequency range that includes a fundamental lowerVHF frequency f. In block 416, a resonator 274 is coupled to theelectrode 204, the resonator having a resonant frequency F which is anodd harmonic of the fundamental frequency f, so as to produce VHF powerat the odd harmonic, using the plasma in the chamber as a non-linearmixing element. In block 418, an individual center return path isprovided through a counterelectrode (e.g., the ESC electrode 226 of FIG.6) for the harmonic frequency F, by providing the bandpass filter 252coupled to ground. In block 420, an individual center return path isprovided through the counterelectrode (226 of FIG. 6) for thefundamental VHF frequency f, by providing the bandpass filter 254coupled to ground. In block 422 of FIG. 12, edge return paths throughthe side wall for both frequencies f and F by grounding the side wall202 of FIG. 6. In block 424 of FIG. 12, the controller 270 adjusts theimpedance of the F center return path relative to the impedance of the Fedge return path to promote current flow at F to the center return path,by adjusting the variable bandpass filter 252. In block 426, thecontroller 270 adjusts the impedance of the f edge return path relativeto the impedance of the f center return path to promote current flow atthe f frequency to the side wall, by adjusting the variable bandpassfilter 254. In block 428, the controller 270 improve plasma ion densitydistribution uniformity, by controlling the ratio of VHF power at (ornear) the fundamental f to VHF power at harmonic F. This is accomplishedby controlling the proportion of power up-converted from f to F. Thisproportion is controlled by controlling the difference between thevariable output frequency of the VHF generator and the fundamentalfrequency f. As the generator output frequency approaches closer to thefundamental, the proportion of VHF power produced by the variablefrequency generator 240 converted to the (third) harmonic F increases,for example. The maximum ratio VHF power at F to VHF power at f isattained when the generator frequency equals the fundamental f. The stepof block 428 may be carried out in order to reduce a center-high plasmaion density distribution by decreasing the ratio of VHF power at Frelative to VHF power at f (block 430 of FIG. 13). Or, the step of block428 may be carried out to reduce an edge-high plasma ion densitydistribution nonuniformity by decreasing the ratio of VHF power at the ffrequency relative to VHF power at F (block 432 of FIG. 13).Alternatively or in addition to the step of block 428, the controller270 may improve uniformity by adjusting the impedances of the center andedge return paths of either or both F and f to either: (a) channel morecurrent toward the edge in order to suppress a center-highnon-uniformity or (b) channel more current toward the center to suppressan edge-high non-uniformity, by adjusting the respective bandpassfilters 252, 254.

The use of an electrostatic chuck 218 facilitates high rates of heattransfer to or from the wafer 220, even at very low (mT) chamberpressures where heat transfer is poor without an electrostatic chuck.This feature enables the vacuum pump 228 to be a very powerful turbopump to run chamber recipes calling for extremely low chamber pressures.These features, in combination with the VHF power sources 240, 242 thatcan produce very low to very high plasma ion densities (e.g., 10̂9 to10̂11 ions/cc), provide a novel capability of low chamber pressure (inthe mT range), high plasma ion density (in the 10̂10 to 10̂11 ion/ccrange) at high bias or high heat load while maintaining complete controlof wafer temperature and plasma ion density distribution uniformity.These features, which are contained in the reactors of FIGS. 1-6,fulfill the needs of certain processes such as dielectric etch plasmaprocesses and plasma immersion ion implantation processes that imposehigh heat load while requiring low chamber pressure and high plasma iondensity. However, these reactors are capable of performing across a widerange of chamber pressure (mT to Torr), a wide range of wafer heat loadand a wide range of plasma ion density (e.g., 10̂9 to 10̂11 ions/cc).Therefore, the reactors of FIGS. 1-6 may also be employed in carryingout other processes at either high or low chamber pressure and at eitherhigh or low plasma ion density, such as plasma enhanced chemical vapordeposition (PECVD), plasma enhanced physical vapor deposition (PEPVD),plasma doping and plasma enhanced materials modification.

While the foregoing is directed to embodiments of the invention, otherand further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. A method of processing a workpiece in a plasma reactor chamber havingelectrodes including at least a ceiling electrode and a workpiecesupport electrode, said method comprising: coupling respective RF powersources of respective VHF frequencies f1 and f2 to one of: (a)respective ones of the electrodes, and (b) a common one of theelectrodes, where f1 is sufficiently high to produce a center-highnon-uniform plasma ion distribution and f2 is sufficiently low toproduce a center-low non-uniform plasma ion distribution; adjusting aratio of an RF parameter at the f1 frequency to the RF parameter at thef2 frequency so as to control plasma ion density distribution, said RFparameter being one of: (a) RF power, (b) RF voltage, and (c) RFcurrent.
 2. The method of claim 1 wherein f1 is greater than about 110MHz and f2 is less than about 90 MHz.
 3. The method of claim 1 whereinsaid adjusting comprises: reducing a center-high plasma ion densitydistribution by decreasing the ratio of the RF parameter at the f1frequency relative to the RF parameter at the f2 frequency.
 4. Themethod of claim 1 wherein said adjusting comprises: reducing anedge-high plasma ion density distribution nonuniformity by decreasingthe ratio of the RF parameter at the f2 frequency relative to the RFparameter at the f1 frequency.
 5. The method of claim 1 furthercomprising: providing respective center ground return paths for RFcurrent passing directly between the ceiling electrode and the workpiecesupport electrode for the frequencies f1 and f2 elements; providing anedge ground return path for each of the frequencies f1 and f2.
 6. Themethod of claim 5 further comprising: adjusting the impedance of thecenter ground return path corresponding to the frequency f1 so as toincrease or decrease the tendency of the RF power at f1 to produce acenter-high non-uniformity in plasma ion density distribution.
 7. Themethod of claim 5 further comprising: adjusting the impedance of thecenter ground return path corresponding to the frequency f2 so as toincrease or decrease the tendency of the RF power at f2 to produce acenter-low or edge-high non-uniformity in plasma ion densitydistribution.
 8. The method of claim 5 further comprising: adjusting theimpedance of at least one of the center ground return paths so as tocontrol plasma ion density distribution.
 9. The method of claim 8wherein said adjusting the impedance is performed so as to increase thecurrent in the edge ground return path to suppress a center-highnon-uniformity.
 10. The method of claim 8 wherein said adjusting theimpedance is performed so as to increase the current in the centerground return path to suppress an edge-high non-uniformity.
 11. Themethod of claim 1 wherein: said coupling comprises coupling the RF powersource of the frequency f1 to the ceiling electrode of the chamber whilecoupling the RF power source of the frequency f2 to the workpiecesupport electrode.
 12. The method of claim 1 wherein: said couplingcomprises coupling the RF power source of the frequency f1 and the RFpower source of the frequency f2 to the ceiling electrode.
 13. Themethod of claim 1 wherein said coupling comprises coupling the RF powersource of the frequency f1 to a first ceiling electrode and coupling theRF power source of the frequency f2 to a second ceiling electrode thatis concentric with the first ceiling electrode.
 14. The method of claim1 wherein said coupling comprises: generating the RF power of frequencyf1 from a fundamental frequency f0 for which f1 is an harmonic using aresonator tuned to f1 and using the plasma in the chamber as anon-linear mixer.
 15. The method of claim 1 further comprising:adjusting the plasma ion density distribution by adjusting a gap betweenthe workpiece and the ceiling electrode.
 16. The method of claim 1wherein said respective RF power sources are one of: (a) continuous, (b)pulse, (c) simultaneous, (d) sequential.
 17. A method of processing aworkpiece on a workpiece support in a plasma reactor chamber havingelectrodes including a ceiling electrode, a workpiece support electrodeand a variable workpiece-to-ceiling gap, comprising: coupling respectiveRF power sources of respective VHF frequencies f1 and f2 to one of: (a)respective ones of the electrodes, (b) a common one of the electrodes;selecting a workpiece-to-ceiling gap such that f1 is sufficiently highto produce a center-high non-uniform plasma ion distribution and f2 issufficiently low to produce a center-low non-uniform plasma iondistribution; and adjusting a ratio of an RF parameter at the f1frequency to the RF parameter at the f2 frequency so as to controlplasma ion density distribution, said RF parameter being one of: (a) RFpower, (b) RF voltage, (c) RF current.
 18. The method of claim 17further comprising: isolating a process region between said workpiecesupport and said ceiling electrode from asymmetrical features in saidchamber by providing an annular confinement element between the processregion and the asymmetrical features in said chamber and extending froma side of the workpiece support to establish a confinementelement-to-side wall gap restriction.
 19. The method of claim 18 furthercomprising coupling said ring to RF ground to provide an edge RF groundreturn path.
 20. The method of claim 17 wherein f1 is greater than about110 MHz and f2 is less than about 90 MHz.
 21. The method of claim 17wherein said adjusting comprises: reducing a center-high plasma iondensity distribution by decreasing the ratio of the RF parameter the f1frequency relative to the RF parameter at the f2 frequency.
 22. Themethod of claim 17 wherein said adjusting comprises: reducing anedge-high plasma ion density distribution nonuniformity by decreasingthe ratio of the RF parameter the f2 frequency relative to the RFparameter at the f1 frequency.
 23. The method of claim 17 furthercomprising: providing respective center ground return paths for RFcurrent passing directly between the ceiling electrode and the workpiecesupport electrode for the frequencies f1 and f2; providing an edgeground return path of said chamber for each of the frequencies f1 andf2.
 24. The method of claim 23 further comprising: adjusting theimpedance of the center ground return path corresponding to thefrequency f1 so as to increase or decrease the tendency of the RF powerat f1 to produce a center-high non-uniformity in plasma ion densitydistribution.
 25. The method of claim 23 further comprising: adjustingthe impedance of the center ground return path corresponding to thefrequency f2 so as to increase or decrease the tendency of the RF powerat f2 to produce a center-low or edge-high non-uniformity in plasma iondensity distribution.
 26. The method of claim 23 further comprising:adjusting the impedance of at least one of the center ground returnpaths so as to control plasma ion density distribution.
 27. The methodof claim 17 wherein: said coupling comprises coupling the RF powersource of the frequency f1 to the ceiling electrode of the chamber whilecoupling the RF power source of the frequency f2 to the workpiecesupport electrode.
 28. The method of claim 17 wherein: said couplingcomprises applying RF power of the frequency f1 and power of thefrequency f2 to the ceiling electrode.
 29. The method of claim 17wherein said coupling comprises coupling the RF power source of thefrequency f1 to a first ceiling electrode and coupling the RF powersource of the frequency f2 to a second ceiling electrode that isconcentric with the first ceiling electrode.
 30. The method of claim 17wherein said coupling comprises: generating the RF power of frequency f1from a fundamental frequency f0 for which f1 is a harmonic using aresonator tuned to f1 and using the plasma in the chamber as anon-linear mixer.
 31. The method of claim 17 further comprising:adjusting the plasma ion density distribution by adjusting a gap betweenthe workpiece and the ceiling electrode.
 32. The method of claim 17wherein said respective RF power sources are one of: (a) continuous, (b)pulse, (c) simultaneous, (d) sequential.